SD 144 
.115 C7 

Copy 1 ~ -zDc Taniversttis of Cbicaao 

FOUNDED BY JOHN D. ROCKEFELLER 



THE CLIMAX FOREST OF ISLE ROYALE 

LAKE SUPERIOR, AND ITS 

DEVELOPMENT 



A DISSERTATION 

SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL 

OF SCIENCE IN CANDIDACY FOR THE DEGREE 

OF DOCTOR OF PHILOSOPHY 

(department of botany) 



BY 

WILLIAM S. COOPER 



Reprinted from 

The Botanical Gazette, Vol. LV, No. i 

Chicago, 1913 



Ube mniversitp ot Cbtcaao 

FOUNDED BY JOHN D. ROCKEFELLER 



THE CLIMAX FOREST OF ISLE ROYALE 

LAKE SUPERIOR, AND ITS 

DEVELOPMENT 



A DISSERTATION 

SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL 

OF SCIENCE IN CANDIDACY FOR THE DEGREE 

OF DOCTOR OF PHILOSOPHY 

(department of botany) 



BY , -y-^ 

WILLIAM S. COOPER 



Reprinted from 

The Botanical Gazette, Vol LV, No. i 

Chicago, 1913 



.WsCn 






VOLUME LV NUMBER i 



THE 

Botanical Gazette 

JANUARY 1 9 13 

THE CLIMAX FOREST OF ISLE ROYALE, LAKE 
SUPERIOR, AND ITS DEVELOPMENT. I 

CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 165 

William S. Cooper 

(with map and fourteen figures) 
Introduction 

Eastern North America north of Florida and Mexico is divided 
into two great phytogeographic regions, the eastern deciduous 
forest and the northeastern conifer forest. In each of these a 
number of hnes of succession may be traced, all those of a region 
leading to a certain forest type as the final or climax stage. This 
final type in its large features is determined by cKmate, and is 
much the same throughout the region which it dominates. In the 
eastern deciduous forest the climax type is made up of Acer sac- 
charum Marsh (sugar maple) and Fagus grandifolia Ehrh. (beech), 
with the addition of various other species in some portions of the 
country. The nature of the climax forest of the northeastern 
conifer region has not hitherto been determined. 

Isle Royale Hes just within the limits of the northeastern conifer 
region, barely within, for one of the farthest outposts of the decidu- 
ous forest is located on its southwestern end, where there is a 
considerable area dominated by the sugar maple, in mixture with 
more northern trees. Except for the maple and a few of its com- 
panion species, the flora of Isle Royale belongs strictly to the north- 
eastern conifer region. 



2 BOTANICAL GAZETTE [january 

The purpose of the present work was to determine the cHmax 
forest of Isle Royale, its composition^ and character, and to trace 
the various Knes of succession leading to it. It is thus a successional 
study of a small component portion of the northeastern conifer 
forest. 

At the beginning of the investigation Isle Royale was selected 
as a field of study because it shows transitional features between the 
two great forest regions, my original purpose being to devote 
particular attention to the relations between the conifers and the 
maple. Circumstances made it impossible to give adequate study 
to the region dominated by the latter, therefore the work developed 
into an investigation of the balsam-birch-spruce forest (the north- 
eastern climax) and its attendant successions. For a study of the 
northeastern conifer forest a more centrally located area might 
have been preferable; for instance, at some point midway between 
Lake Superior and James Bay. It will be shown, however, that 
Isle Royale affords a very fair sample of the forest growth of the 
northeastern region. It also possesses certain very important 
advantages which would be lacking in a more centrally located 
area. Because of its insular position the forest has been less hable 
to destruction by fire, and the many bays and channels separating 
various portions of the main island and the outlying islets have 
served as effective barriers against its spread. Though they have 
occurred many times during the island's history, it is certain that 
fires have been far less frequent and destructive here than upon 
the mainland. The forest may thus be studied in a condition that 
is as near to being undisturbed as will be found anywhere. Com- 
parative freedom from the destructive agency of man is a second 
advantage. Again, the island has had a simple physiographic 
history during the present vegetative cycle, and thus the relation 
of vegetation to physiography may be the more readily made out. 
Further, the proximity of the lake shores permits the observation 
of the earliest stages in the establishment of vegetation upon the 
rock surfaces, these stages being frequently absent or poorly 
developed in an inland locaKty. Finally, the fact that the field 
of study is an island gives definiteness to the area covered b}^ the 
investigation. 



1913] COOPER— ISLE ROYALE 3 

Headquarters were established at the Park Place Hotel on Rock 
Harbor (sec. 4, T. 66 N., R. 33 W.), and field investigations were 
carried on upon the island during the summers of 1909 and 1910. 
Most of the detailed work was done in the vicinity of Park Place, 
but the coast was visited from Hawk Island to Blake Point on 
the northwest, and from Blake Point to the head of Siskowit Bay 
on the southeast. Excursions into the interior were made from 
various points along this stretch of shore. 

The study was undertaken at the suggestion of Dr. Henry C. 
CowLES of the University of Chicago. I wish to express my appre- 
ciation of his invaluable assistance and co-operation, freely given 
at all times during the progress of the investigation. I desire also 
to extend my thanks to Dr. M. L. Fernald of the Gray Herbarium 
of Harvard University, who determined all doubtful sperma- 
tophytes and pteridophytes, and to Miss Edith A. Warner of 
Brooklyn, N.Y., who determined the mosses collected upon the 
island, about 80 in number. The nomenclature of the pteri- 
dophytes and spermatophytes is that of the seventh edition of 
Gray's Manual. 

PREVIOUS BOTANICAL WORK UPON ISLE ROYALE 

A limited amount of botanical work of taxonomic and ecological 
nature has been done upon Isle Royale. In 1848 W. D. Whitney, 
acting as naturalist for a government exploring party, made a 
brief Hst of plants found upon the island, which was published in 
the report of the expedition (24) in 185 1. In 1890, according to 
Adams (4), F. E. Wood made a collection of plants in the vicinity 
of Rock Harbor and presented them to the University of Michigan. 
In 1901 W. A. Wheeler (58) pubhshed a hst of noteworthy 
species, reporting for the first time the strange occurrence of 
Fatsia horrida (devil's club) upon the island. 

In 1904 and 1905 Isle Royale was visited by parties from the 
Museum of the University of Michigan, both equipped for ecologi- 
cal work among plants and animals. The first, under the leader- 
ship of Dr. A. G. Ruthven (3), spent three weeks upon the island, 
after a month's work in the Porcupine Mountains of the northern 
peninsula of Michigan. Their explorations were confined to the 



4 BOTANICAL GAZETTE [january 

southwestern end of the island. In respect to the vegetation the 
results of this expedition consist in scattered ecological notes and a 
list of plants including only 91 species (49). 

The second party, headed by Dr. C. C. Adams, devoted all its 
time (about six weeks) to Isle Roy ale, and the resulting report is 
incorporated in a volume of more than 400 pages (4) . The botani- 
cal work was done by Holt (33), whose report comprises a ten- 
page account of the plant societies, and an annotated list of lichens, 
mosses, ferns, and seed plants, including 364 species. There is 
also much of ecological value to be found in the sections by Adams, 
and the report by Gleason upon the ecology of the invertebrates 

(29). 

Two papers by the present writer (12, 12a) should also be men- 
tioned, as they were suggested by observations upon Isle Royale. 

TOPOGRAPHY AND PHYSIOGEAPHIC HISTORY 

Isle Royale is situated in the northwestern part of Lake Superior 
in lat. 48° N., long. 89° W., about 25 km. distant from Thunder 
Cape, which is the nearest point of the Canadian mainland. The 
island is elongated, extending northeast and southwest, and its 
dimensions are 72 km. by 14 km. at the widest part. It is formed 
of several parallel ridges which are made by the resistant centers of 
successive outcrops of a series of Keweenawan lava flows. These 
dip southeastward at angles varying from 5° to 40°. The southeast 
slopes of the ridges are gentle, corresponding with the dip of the 
beds, while the northwest faces are steep and broken, often pre- 
cipitous. Several of them extend into the lake at either end of 
the island as promontories or rows of small islands (fig. i). The 
largest, the Greenstone Range, stretches the whole length of the 
island, and is continued northeastward in Passage Island and Gull 
Rocks. At several points it reaches an altitude of more than 150 m. 
above the lake level. Between the ridges are narrow valleys, 
corresponding with the less resistant peripheral portions of the 
flows and the sedimentary layers that are interbedded with them. 
These contain many lakes, and where submerged at the ends of the 
island form narrow fiord-like harbors and channels. The drainage 
is well adjusted to structure, the streams flowing along the strike 



I9I3] 



COOPER—ISLE ROY ALE 



of the less resistant beds, entering the lake at the ends of the 
valleys, or occasionally through narrow cross valleys, most of 
which are due to faults. 

The quaternary history of Isle Royale is briefly as follows : At 
the beginning of the glacial period the topography, produced 
during a long period of subaerial erosion, was essentially as now. 
The ice completely covered the island, moving southwestward 
nearly with the strike of the beds, but wrought only shght modi- 
fications in the topography. Rock basins were excavated in the 




Fig. I.— Southeast across Scovill Point and the outer islands from the slope of the 
Greenstone Range: Tobin's Harbor in the foreground; Rock Harbor beyond; Lake 
Superior in the distance. 

valleys and many surfaces were smoothed and striated. Roches 
moutonnees are common. Of the httle drift that was left behind 
most was dropped upon the southwest end, and practically all 
has been rehandled by the waters of the successive postglacial lakes. 
Upon the retreat of the ice, Isle Royale was left entirely sub- 
merged beneath, the waters of Lake Duluth. The remaining 
history records a gradual emergence corresponding with the 
repeated changes of the water level as the lake found successively 
lower outlets. That this emergence was frequently interrupted 
is shown by the beaches, sea cliffs, and wave-cut terraces that 
occur at various altitudes, corresponding with similar ones along the 



6 BOTANICAL GAZETTE [januaey 

mainland coast. These indicate periods when the water level was 
stationary for a considerable time. According to Lane (36) the 
present shore Hne is more strongly marked than any at higher 
levels. "Nor is it surprising," Lane remarks, "that the lake 
level should now be tolerably constant, for Lake Superior now 
drains over a rock threshold." In comparatively recent post- 
glacial time (since the formation of the very recent Nipissing beach) 
tilting occurred in the Lake Superior region, with uplift northward. 
This must have modified more or less the drainage conditions 
upon Isle Royale. It is important to bear in mind the self-evident 
fact that never since its first emergence from the waters of Lake 
Duluth has Isle Royale been connected with the mainland. 

For further geologic and physiographic details the reader is 
referred to Lane's report (36), to which I am indebted for the 
material for this brief sketch ; and also to Adams (4) , who discusses 
the physiographic history of the island with considerable fulness. 
Adams also gives much valuable data concerning the influence of 
the lake storms and surface currents upon the biota of the island. 

PHYSIOGRAPHIC AGENCIES NOW AT WORK 

The agencies that are now modifying the surface of the island, 
which are of course the same that have been active throughout 
its history, may be considered under two heads. 

Among the destructive agencies, weathering is of the greatest 
importance in its influence upon vegetation. It is most evident 
upon the steep northwest slopes of the higher ridges. Here there 
are somewhat extensive talus piles lying at the bases of cliffs, or 
in some cases occupying the whole slope, the cliff having been 
buried by the accumulation of fragments. In many places the 
talus is fully clothed with climax forest, in others the fragments 
are bare or merely lichen covered. The results of weathering are 
evident also upon the bare rock shores, where scales and plates are 
seen to have been split from the rock surfaces through the agency 
of temperature changes. Very important, though effectually 
concealed, is the chemical action which is going on beneath the 
humus carpet that covers most of the island's surface. Between 
the humus and the bed rock there is nearly everywhere a layer of 



1913] COOPER— ISLE ROYALE 7 

small rock fragments mixed with organic matter. Most of these 
fragments are so decomposed that they can be cut easily with a 
knife. The bed rock itself frequently shows the effect of chemi- 
cal action. Vegetation is here seen as an important physiographic 
agent. 

Stream erosion is of trifling importance upon Isle Royale because 
of the small size and low gradient of most of the streams and their 
freedom from transported materials, necessary as agents of abrasion. 

Wave erosion is the most conspicuous of destructive agencies. 
The surf is actively cutting into the land, and the shore features at 
the present lake level are very pronounced. At many points along 
the southeast coast the normally gentle slope of the shore has been 
transformed into terrace and chff. In some parts of the abrupt 
northwest shore the waves are undermining the chmax forest itself. 
In connection with erosion by waves should be mentioned the work 
of ice, the precise effects of which could not be determined in sum- 
mer study. 

Under the head of constructive agencies come deposition 
by streams, waves, currents, and vegetation. The only notable 
instances of stream deposition are the few deltas, the materials for 
which were derived largely from the glacial drift and the products 
of wave erosion at former levels. The subject is treated further 
under the head of "Delta swamp succession." The fragments 
eroded by waves are deposited in the form of beaches and bars, 
in coves and harbors. Shore currents are effective in transporting 
the material, and in sweeping the finest into sheltered bays, where 
it is dropped in the quiet waters. The work of vegetation consists 
in the formation of peat and humus. Plant life here again appears 
as a physiographic agent of great importance. 

CLIMATE 

Adams (4, pp. 41-44) has described in some detail the cHmate 
of the general region, his data being obtained from the records of 
the Weather Bureau at Port Arthur. The following summary 
(table I) is derived partly from Adams' account and partly from 
more recent data from Port Arthur obtained through the courtesy 
of the Canadian Weather Service. 



BOTANICAL GAZETTE 



[JANUARY 



TABLE I 

Normal temperature at Port Arthur for the 20 years 1888-1907 





Jan. 


Feb. 


March 


April 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Yr. 


°c 


-139 


-13-7 


-7.2 


1-7 


7-7 


13.6 


16.8 


iS-6 


II. 6 


5-3 


-2.8 


-9.8 


2.1 



The average maximum temperature for the ten years 1896- 
1905 was 30.1° C; the average minimum for the same period was 
— 34.9° C. The mean monthly temperature was below 0° C. for 
five months, and the mean monthly minimum below 0° C. every 
month except June, July, and August. The growing season is thus 
short, including about four months, or even less, between the 
middle of May and the middle of September. The long northern 
period of daylight compensates somewhat for the short season. 

TABLE II 





Normal precipitation at 


Port Arthur 


FOR 


THE 


20 YEARS 


1888 


-1907 




Jan. 


Feb. 


Mar. 


April 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Year 


cm 


1.77 


I-3I 


2.44 


4.18 


5-56 


7.67 


9.82 


7-77 


8.23 


6.49 


3-iS 


1.77 


60. 16 



It is here seen that the greater part of the precipitation takes 
place during the growing season; 39.05 cm., or more than three- 
fifths of the total, occurs during the months May-September. The 
■snowfall is rather light; during the six years 1 900-1 905 the average 
was 61.03 cm. Reduced to water this amounts to a precipitation 
of 6.1 cm., which is about one-tenth of the average total for that 
period. 

The evaporation rate is doubtless low because of the low 
temperature, but there are no data available. This and the rela- 
tively abundant precipitation during the growing season seem 
adequate to account for the extreme mesophytism of the forests 
of the region. 

Since 1906 temperature and precipitation records have been 
kept during the season of navigation by Captain Malone at the 
Hghthouse on Menagerie Island. This is one of the Isle Royale 
archipelago situated 4 km. distant from the nearest point of the 
main island. During the summer of 1 910 I kept thermograph and 
rain gauge records at Park Place. There is therefore opportunity 



I9I3] 



COOPER— ISLE ROY ALE 



for a partial comparison of the insular climate with that of the 
mainland. Summarization of the records for Port Arthur, Park 
Place, and Menagerie Island gives the following results: 



TABLE III 
Temperatures ° C. 



Port Arthur . 



Park Place . 



Menagerie Island . 



June 



Mean maximum 
Mean minimum 
Mean daily range 

Mean maximum 
Mean minimum 
Mean daily range 

Mean maximima 
Mean minimum 
Mean daily range 



July 



23-9 
II. 7 
12. 2 

21 .4 

12.4 

9.0 

18.8 

10.5 

8.3 



August 



21.3 
10. o 
II-3 

21.4 

12.9 

8-5 

19.4 

I3-I 

6.3 



From this table we see that the maxima upon Isle Royale 
are lower than upon the mainland, during the summer at least. 
Menagerie Island, most under the lake's influence, is the lowest, 
and Park Place, upon the main* island, is intermediate. We may 
infer that as a result of lower temperatures the evaporation is less 
upon Isle Royale. Adams (4, P- 44) suggests that insular location 
and imperfect drainage probably operate to reduce evaporation. 
The table also shows that the Isle Royale chmate is characterized 
by less pronounced daily temperature changes than is that of the 
mainland. The daily range is shown to be uniformly greatest at 
Port Arthur, intermediate at Park Place, and least at Menagerie 

Island. 

A further comparison (table IV) brings out the fact that the 
proximity of the lake retards the opening of the growing season, but 
also that the same factor prolongs it into the fall. 

TABLE IV 

Mean monthly temperatures (° C.) May-Nov.; average 1906-1909 





May 


June 


July 


August 


Sept. 


Oct. 


Nov. 


Port Arthur 


7-9 
4-3 


13-6 
8.9 


16.9 
II. 6 


iS-9 
14.4 


II. 6 
12.6 


5-7 
6.8 


-1-7 




1.9 








lO BOTANICAL GAZETTE [January 

Evidencing the retardation of the season is the fact that ice 
frequently remains in sheltered places on the northwest coast of 
the island into July. I have seen a deserted mine shaft filled solid 
with ice on July 4. Foster and Whitney report ice "under the 
shade of crags, and among the thick evergreen swamps of white 
cedar." 

In precipitation there is apparently not much difference between 
Port Arthur and Isle Roy ale, at least during the growing season. 
As far as the records go the mainland has a slight advantage. 

TABLE V 

Average precipitation May-Nov., i 906-1 909 

Port Arthur 48 . 00 cm. 

Menagerie Island 43-74 cm. 

From the foregoing data, which unfortunately are rather frag- 
mentary, it appears that there is at present no ground for concluding 
that the island climate is effective in producing a more mesophytic 
type of vegetation than that of the mainland, or vice versa. The 
lower evaporation rate upon the island, due to lower temperature, 
is balanced by a less precipitation during the growing season; and 
the growing season, although retarded in its commencement upon 
the island, is apparently as long as upon the mainland. The 
question cannot fully be settled without fuller precipitation records 
from Isle Royale and a determination of the actual evaporation 
rates in the two localities. The data presented, however, indicate, 
so far as they go, that the insular position of the field of study does 
not seriously affect its value as a fair sample of the region dominated 
by the northeastern conifer forest. 

Part I. — The climax forest 

The forest that completely clothes the surface of Isle Royale, 
with the exception of a part of the bog areas, some limited stretches 
of xerophytic character, and the small portion dominated by the 
maple, is made up largely of three trees: Abies balsamea (L.) Mill 
(balsam fir), Betula alba L. var. papyrifera (Marsh) Spach (paper 
birch), and Picea canadensis (Mill) BSP (white spruce). The 
studies embodied in the present paper show that this type is the 



1913] COOPER— ISLE ROY ALE 1 1 

climax forest of that portion of the northeastern conifer region 
under consideration; in other words, that upon Isle Royale it is 
the final and permanent vegetational stage, toward the establish- 
ment of which all the other plant societies are successive steps. 
It is the "climatic" forest of the region, permanent while the 
climate remains essentially as now. 

The evidence in support of this conclusion lies along four lines: 
(i) extreme mesophytism of the forest; (2) its uniformity of 
development; (3) all successions lead to it; (4) maintenance of 
equilibrium. These lines of evidence will now be considered in 
order. 

1. Mesophytism. — The balsam-birch- white spruce forest is the 
most mesophytic of all the plant societies of the island. The truth 
of this statement will appear during the discussion of other points 
and so no further treatment is necessary here. 

2. Uniformity of development. — In all places where it 
occurs, whether upon rock surfaces or reclaimed bogs, upon the 
part most recently emerged from the lake or upon the highest 
ridge, the dominant forest is essentially uniform in character. The 
tree species are the same, and* they bear everywhere the same 
relations to each other. 

3. All successions lead to the balsam-birch- white spruce 
forest. — In a later portion of this paper the various successions 
are treated in detail, and it is shown that all end with the establish- 
ment of the balsam-birch-white spruce forest. In other words, 
those phases of the vegetation that are not uniform in character 
with the main forest mass are plainly tending toward uniformity. 
The successions upon Isle Royale may be classified as follows : 

A. Primary successions 
I. Xerarch^ successions 

1. The rock shore succession 

2. The beach succession 

' The terms xerarch and hydrarch are here used for the first time, for the purpose 
of indicating a natural and important classification of plant successions. The former 
is applied to those successions which, having their origin in xerophytic habitats, such 
as rock shores, beaches, and cliffs, become more and more mesophytic in their succes- 
sive stages; the latter to those which, originating in hydrophytic habitats such as 
lakes and ponds, also progress toward mesophytism. 



12 BOTANICAL GAZETTE [January 

II. Hydrarch successions 

1. The bog succession 

2. The delta swamp succession 
B. Secondary succession 

The burn succession 

4. Maintenance of equilibrium. — ^It has been possible to 
state with brevity the three points that have so far been presented; 
indeed, to one visiting the island they are almost self-evident. The 
validity of the fourth is not so plainly to be seen. 

Both observational and experimental studies have shown that 
the balsam-birch-white spruce forest, in spite of appearances to the 
contrary, is, taken as a whole, in equilibrium; that no changes 
of a successional nature are taking place within it. Superficial 
observation would be Hkely to lead to exactly the opposite conclu- 
sion. In the presence of the other good evidences of permanence 
outhned in the preceding paragraphs, it became necessary to seek 
for an explanation of the seeming condition of rapid change that was 
apparently so plain in the forest. The solution was found in the 
course of a detailed study which included (i) the characteristics 
and Hfe history of each tree species and of certain of the lower 
forms which were of importance, and (2) all the processes and 
changes brought about through the interrelations of the forest 
species, discovered by intensive study of a number of Hmited areas 
of definite size (quadrats), with every available source of evidence 
laid under contribution; in other words, an attempt at a thorough 
investigation of the dynamics of the forest. In the following dis- 
cussion the results will be given under three heads: I. Description 
of the forest;, 11. Studies of individual species; III. Quadrat 
studies. 

I. description or the forest 

For the sake of concreteness I have selected a definite locality 
for description, bearing in mind, however, that such a thing as a 
piece of forest of Hmited extent which is "typical" of the growth of 
a region hardly exists. 

Smithwick Island (sec. 4, T. 66 N., R. 33 W.) was the selection 
for this description and for particular study, because the outer 
row of small islands inclosing Rock Harbor had the appearance of 



1913] COOPER— ISLE ROYALE 13 

having been least disturbed by accidental conditions such as fires; 
and of these islands Smithwick was the most conveniently situated. 
So primeval and luxuriant is the aspect of the forest here that at 
first it seemed almost safe to assume that the island had never been 
burned over since its emergence from the lake. I found, however, 
in one place, at a depth of one-third of a meter, a layer of carbona- 
ceous material with fragments of charcoal. It is certain then that 
fire, to an unknown extent, has entered into the history of the 
island, notwithstanding the many indications to the contrary. 
Nevertheless, granting that the island may have been burned over 
at some time, it is plain that the forest has long since returned to 
its natural condition and may fairly be taken as a suitable area 
for the study of the climax state. We may be sure that the forest 
on Smithwick Island has not been disturbed for many hundreds 
of years at least, and this is not often the case on the main Isle 
Roy ale. 

There is one somewhat abnormal feature of the conditions 
surrounding these outer islands that should be mentioned, namely, 
that the exposure to the strong lake winds is greater than on the 
main island, and that the death-rate among the trees is thereby 
increased, and not always proportionally among the different 
species. On the whole, though, this added exposure merely inten- 
sifies certain processes that are in operation everywhere, and thus 
renders them easier of observation. 

The average elevation of Smithwick Island is about 7 m. The 
southwest one-third was thoroughly burned over about 15 years 
ago. The forest covers the imburned portion almost completely, 
being bordered along most of the Rock Harbor side by a narrow 
shingle beach, and on the lakeward edge by a belt of bare sloping 
rocks, frequently interrupted by broken or precipitous sea cliffs. 

Seen from Rock Harbor the forest has the following appearance, 
and this description will apply fairly well to the climax forest of 
Isle Royale in general (fig. 2). The first impression is of great 
density, the thick fohage extending to the ground at the edge of the 
forest, allowing no view into the interior. The sky line is ragged, 
made up of an irregular combination of sharp points and rounded 
curves, due to the mixture of broad-leaved trees and conifers. 



14 ' BOTANICAL GAZETTE [january 

Above the general level of the treetpps tower occasional very old 
white spruces, conspicuous features in spite of or rather on account 
of their fewness. The paper birches make considerable show by 
reason of their thick tops, often appearing to compose at least half 
of the forest, but in reality not much more abundant than the 
spruces. The balsams are plainly very abundant, and are actually 
even more so than they seem, since many small ones are hidden by 
.other trees. There are a few large specimens approaching the 
spruces in size, and thick groves of medium-sized trees are just 




Fig. 2. — Exterior view of the climax forest upon one of the row of islands bound- 
ing Rock Harbor on the southeast: two tall white spruces at the right; a group of 
balsams at the left; several large birches. 

visible, their spirelike tips appearing in dense clusters among the 
birch tops. The forest toward the harbor is bordered by a belt 
where Alnus crispa (Ait.) Pursh (green alder) is common, fUHng 
in the gaps between the trees. In this region Pyrus americana 
(Marsh) DC (mountain ash) is also frequent, and Thuja occidentalis 
L. (arbor vitae) is occasional. 

Upon entering the forest we seem in many places to be in the 
midst of a dense growth of nearly pure balsam. The individuals 
of this species are of all sizes, and there is a pronounced tendency 
among them to grow in close groups. The small trees (roughly 



19^3 ] 



COOPER— ISLE ROY ALE 



IS 



those 7 m. high and under) are greatly in excess of the larger ones. 
There are also numerous dead and dying specimens, almost always 
small ones, some of the dead trees showing evidence of having suc- 
cumbed very recently, the needles not yet having dropped off. 
The occasional large trunks of the birches are conspicuous objects, 
but young ones are not numerous. It is often difficult to find a 
single spruce, unless one has 
carefully estimated from the 
exterior the position of one of 
the conspicuous old specimens. 
Young spruces are exceedingly 
rare, so that a long search will 
be necessary to discover one. 
The shade in most parts, espe- 
cially under the closely placed 
balsams, is dense, though there 
are frequent partial openings, 
caused principally by windfalls 
(fig. 3). Standing dead trees 
of large size are very rare, but 
fallen trunks in all stages of de- 
composition are numerous, the 
greater number being balsams, 
though the dead birches are 
more conspicuous on account of 
their greater size. 

Shrubby growth is not 
abundant. The areas of not too 
dense shade are often thickly 
populated with Taxus canadensis 
Marsh (ground hemlock) . Other large shrubs that are more or less 
frequent are Alnus crispa (Ait.) Pursh (green alder). Viburnum 
pauciflorum Raf. (high bush cranberry), Sambucus racemosa L. 
(red-berried elder), Lonicera canadensis Marsh (bush honeysuckle), 
Fatsia horrida (Sm.) B. & H. (devil's club), the last abundant in 
one restricted area. 

The herbaceous growth is sparse except in partial openings. 













crj-^^lmp 








''■Jb*'^' '■* i. 






if»-./e^!^' 




■ ~-\ '^.^^'■■.. - 


■ . 1 


*'P'-' .'•'. 


^ 


*'■ 


..%■- 



Fig. 3.^ — -Illustrates conditions result- 
ing from a windfall in the climax forest: 
fallen trunks and young balsams; Smith- 
wick Island. 



i6 BOTANICAL GAZETTE [January 

Most promineht is the association of about eight herbs which is 
so characteristic of the northeastern conifer forest, and in part of 
similar forests over a much wider range. The group includes the 
following: Cornus canadensis li. (bunch-berry), Trientalis americana 
(Pers.) Pursh (star-jflower) , Linnaea horealis L. var. americana 
(Forbes) Rehder (twin-flower), Maianthemum canadense Desf. 
(two-leaved Solomon's seal), Clintonia horealis (Ait.) Raf., Mitella 
nuda L. (mitrewort), Aralia nudicaulis L. (wild sarsaparilla) , 
Coptis trifolia (L.) Salisb. (goldthread). These species are found 
in every part of the Isle Royale climax forest, and many of them 
in the bog forest, bogs, and along the rock shores as well. Others, 
less characteristic and abundant, still occur commonly: Ly co- 
podium annotinum L. (stiff club moss), Z. ohscurum L. (ground pine), 
Phegopteris Dryopteris (L.) Fee (oak fern), Aspidium spinulosum 
(0. F. Miiller) Sw. (shield fern), Poly podium vulgare L. (polypod), 
Cystopteris fragilis (L.) Bernh. (fragile fern), Moneses uniflora (L.) 
Gray (one-flowered Pyrola) , Rihes prostratum L'Her (fetid currant) , 
Epipactis repens (L.) Crantz var. ophioides (Fernald) A. A. Eaton 
(rattlesnake plantain), Oxalis Acetosella L. (wood sorrel), Hahenaria 
ohtusata (Pursh) Richards, Comandra livida Richards. 

By far the most important part of the herbaceous vegetation, 
both quantitatively and ecologically, is the moss contingent. This 
forms a nearly continuous carpet, being absent only where the 
shade is very dense. Three species are chiefly concerned, and these 
are quite equally distributed, one usually being dominant in a 
given spot. Calliergon Schreheri (Willd.) Grout (Hypnum Schreheri 
Willd.) is perhaps the most abundant, and grows in the drier 
places alone, as well as mixed with the other two in general. Hylo- 
comium proliferum (L.) Lindb. usually covers the areas of well 
decomposed humus; while Hypnum crista-castrensis L. seems to 
prefer rotten wood. Next to these in abundance is Hylocomium 
triquetrum (L.) Lindb. 

The humus soil, which is composed largely of moss remains, 
tree waste, and rotten wood, varies in depth from 0.25 to 6 dm., 
the average being perhaps about 3. dm. It rests directly upon the 
smooth rock surface or is separated from it by a loose layer of 
decomposed fragments. 



1913] COOPER— ISLE ROYALE 17 

Returning to the trees, the first conclusion would naturally be 
that we have here a stage in the succession approaching but not 
having reached the final or cHmax condition. The spruces and 
birches appear like relicts, and the balsams, which seem to be of all 
ages, but mostly younger than the trees of other species, are appar- 
ently succeeding them. The seeming probability is that before 
long the birches and spruces will have died out, leaving a pure 
growth of balsam which in the future will succeed itself. Appear- 
ances of this kind have sometimes been considered sufficient to 
prove that succession is in active progress, and there are undoubt- 
edly many cases where the phenomena are not deceptive. In no 
case, however, should the mere appearance of rapid succession be 
admitted as valid evidence until verified by surer methods of study. 
The results of an attempt to use such methods are detailed in the 
two following sections. 

II. STUDIES OF INDIVIDUAL SPECIES 

Abies balsamea (balsam fir). — If it be objected that the forest 
is after all a practically pure stand of balsam, with a mere scatter- 
ing of other species, the following facts will be sufficient answer. 
It is true that in number of individuals, all sizes considered, the 
balsam is greatly preponderant. Of the 254 trees included in the 
six quadrats soon to be described, 78 . 7 per cent are balsam. But 
if we take account only of those trees which may be considered as 
forming the mature stand, the percentage of balsam becomes much 
smaller. Size, not age, is here the proper criterion. Considering 
those trees which are 1.25 dm. and more in diameter, which is a 
rather low limit to set, the proportion is only 56 . 7 per cent. Among 
the larger trees the balsams are still less numerous, making only 
33.3 per cent of those 2.5 dm. and more in diameter (fig. 4). The 
same facts are shown when age is considered instead of size, though 
in a somewhat less striking manner (fig. 5). 

Two causes are responsible for the preponderance of balsam in 
the young growth. First, the seedlings make a successful start 
in almost any sort of situation, provided sufficient light be available. 
Very young seedlings were seen commonly in such diverse situa- 
tions as the following: natural openings in the forest caused by 



i8 



BOTANICAL GAZETTE 



[JANUARY 



windfall, in moss and humus (by far the commonest situation); 
windfall opening in cedar swamp, in moss; rotten logs in forest 



Total: 
254 trees. 



Lar'i X , . fV' 

la r 1 c ( n a ,j^o 

Pice a"" / 

m afiana _ __,' / 
Populus / , 

t rernujoides/ / 
Pljpus . , 

americana / 

Pice a 

canadensisy 

Betula a-lba 
pap4rifera_ 

Abies, 

balsamea 



4.7 Z 



2,8 7o 



10.67. 



Over I.2S dm. 

in diameter: 

67 trees. 



TSZ 



1.57. 



7.5-% 



^% 



zb.^x 



56.7^ 



Over 1.5 dm 
in diameter- 
18 trees. 



5.5f. 



5.5? 



16-7;^ 



i'^X 



iiif. 



Fig. 4. — Composition of the climax forest; according to size of trees 

(infrequent in this region); open bogs in sphagnum; crevices and 
humus-filled depressions on rock shores; burned areas, both 
forested and bare, not abundant ; upper beach among large shingle. 



I9I3] 



COOPER— ISLE ROY ALE 



19 



in partial shade and entirely open, abundant in one locality; sand 
bar across mouth of small stream, abundant. The entire absence 



Lari X ^'^V 

laricina ,gt/ 

Picefl " ' //' 

mar I an a __/ / 

Populus' / ,■ 

tremujotdes / / 

Pqrus / /■ 

americana. _/ / 

Pice a , / 

canadensis _/ 

B etui a aFba"" 

papHriiera 
Abies 

balsamea 



Total : Over 30 qrs: Over 60 yrs-. Over 100 ijrs: 

Z54 trees, 171 trees 6 j trees. u trees. 

IT 



ZA% 



^If 



x.%% 



lO-fcZ 



78.7X 



3.5 






3.5Z 



^At 



l'.7Z 



76! 



1.6?; 



i.fer. 



!■&% .■ 



Ifcfc/. 



41.7^ 



41.7^ 



Fig. S- — Composition of the climax forest; according to age of trees 

of very young seedlings wherever the shade is even moderately 
dense is noteworthy. Later in life the young trees can endure 
severe shading, but for a successful start abundant Kght seems to 



20 



BOTANICAL GAZETTE [jantjary 



be a necessity. The possibility must be admitted that hght (or 
rather radiant energy) is only indirectly the important factor, its 
influence lying in its effect upon the seed bed. Densely shaded 
soils upon Isle Royale are nearly everywhere more or less of the 
nature of peat, low in temperature, soggy with unavailable water, 
and probably Kke peat deficient in certain types of bacterial and 
fungal Hfe. The obvious effects of abundant access of radiant 
energy would be to partially cure the sogginess of the soil and thus 
increase its oxygen content, and to bring about a high soil tempera- 
ture; both of which changes would result in greatly increased 
activity among the various types of soil organisms. Plans to 
carry out some experiments with a view toward determining the 
germination conditions of the balsam and other trees were frustrated 
by the total failure of the seed crop in 1910. 

A second cause, which easily accounts for a considerable part 
of the young growth of balsam, is found in the habit of layering, 
by which that species reproduces abundantly. All the other coni- 
fers of Isle Royale except the pines possess the habit too, but to a 
much less degree. In a previous paper (12) I have described in 
detail the layering habit of the balsam and other conifers, and 
therefore a few words here will be sufficient. In the forest one 
frequently comes upon small groups of young balsams, composed 
often of about half a dozen individuals of various sizes. Upon 
superficial inspection these would easily pass for a cluster of seed- 
Hngs, but if the group be carefully dug up it will be found that the 
young trees are all connected with each other just below the surface 
of the ground. The group comes into existence in the following 
manner. One or more of the earliest branches of a young tree 
(which is sometimes hardly beyond the seedling stage) comes to be 
sHghtly covered with humus and litter, and produces roots. The 
tips then become erect, and taking on radial symmetry are trans- 
formed into miniature trees. By successive layering of branches as 
many as five generations produced in this manner may be included 
in a single group. Large drooping branches of mature individuals 
may layer in the same way, and it is not uncommon to find an old 
trunk surrounded by a circle of daughter trees developed from 
layered branches. The young shoots soon come to depend entirely 



1 913] COOPER— ISLE ROY ALE 21 

upon their own root systems for sustenance, and there is evidence 
that a considerable number of them become independent through, 
the decay of the connecting branch. The habit is so common in 
the Isle Royale forests that a large proportion of the apparent 
balsam seedlings may be accounted for in this way. 

The preponderance of balsam in the young tree growth being 
accounted for, it is now necessary to explain its rapid decrease 
when greater size and age are considered. Several causes combine 
to bring this about. Abundant germination is itself a disadvantage, 
since it results in severe competition, much of the stand undergoing 
suppression and finally death. The species is very susceptible 
to fungus attacks and to diseases of many kinds. Rotten-hearted 
trees are very common. Witches' brooms caused by a rust {Peri- 
dermium) are famihar objects. According to Moore and Rogers 
(41), the HabiHty to fungus attack is greater in pure stands than 
where trees are scattered. The common group habit of the species 
is therefore a disadvantage in this respect. The prevalence of 
heart rot, together with the natural brittleness of the wood, cause 
extreme liability to windfall, and broken trunks are a common 
sight, while uprooted balsams are rare. It is not surprising, in 
view of these facts, that in spite of its proHfic power of germination 
the balsam never reaches the position of dominance in the mature 
stand. In a word, its high birth-rate is balanced by a high rate of 
mortality. 

Betula alba var. papyrifera (paper birch) . — The prominence 
of this species in the mature stand and in the general aspect of the 
forest has been noted, and also its comparative scarcity in the young 
growth. It is certain that the germination of the birch in this 
region is far from proHfic. Very young seedlings were frequently 
seen, and in situations almost as varied as those inhabited by the 
balsam, but never in abundance as in the case of that tree. I 
quite frequently found very small seedHngs in dense shade, but they 
were never more than five or six years old, indicating that conditions 
(probably light supply for photosynthesis) were not favorable for 
continued growth. Opportunity for successful reproduction comes 
usually, as in the case of the balsam, after windfalls (fig. 8). On 
account of its much less prolific germination the birch is far less 



22 BOTANICAL GAZETTE [january 

abundant in such situations than the balsam. Its growth under 
.the same conditions seems to be somewhat faster, however, and so 
the few birches of the windfall area, or some of them, soon overtop 
the balsams and cause the suppression of many of the latter, at the 
same time, with the aid of the balsams, temporarily preventing 
further reproduction of either species. Paper birch has com- 
paratively few and ineffective fungus enemies (Dana i8) and is 
not particularly susceptible to damage by wind, on account of its 
elastic branches and extensive though shallow root system. Even 
when it is broken off in severe storms, as occasionally happens, it 
has a means of recovery in its abihty to send up vigorous sprouts 
from the stump. Occasional clumps of immense birch sprouts 
scattered through the forest are evidence of this power. The most 
effective obstacle to its increase is competition with the balsam 
in its early stages, and here its greater rate of growth gives it a 
shght but important advantage. On the whole' it may be said with 
certainty that its low birth-rate is compensated by a very low 
mortaHty, and it is thus able to maintain itself in making a good 
proportion of the mature stand (figs. 4, 5). 

PiCEA CANADENSIS (white spruce). — This species is ecologically 
much less important than the first two, occurring only sparingly 
in most places; but it attains a greater size than the other trees, 
and is one of the most conspicuous features of the forest. On 
account of its scarcity little could be discovered concerning its 
Hfe-history upon Isle Royal e. From the few seedHngs that were 
observed it seems probable that abundant light is necessary for 
its successful reproduction. According to the United States Forest 
Service (22) it is not a prohfic seed bearer, and has definite seed 
years, which in New England are about eight years apart. All the 
young trees seen were growing in situations where at least fairly 
abundant light was available. It seems probable therefore that the 
white spruce is also largely dependent upon windfalls for its suc- 
cessful reproduction in the virgin forest. It is able to withstand 
severe winds without breaking, as is shown by individuals towering 
conspicuously above the general forest level. It is not particularly 
liable to fungus injury. Birth-rate and mortahty are both low, 
and the species is able to maintain its small proportion in the forest. 



1913] COOPER— ISLE ROYALE 23 

Other trees. — Pyrus americana (Marsh) DC (mountain ash), 
though fairly common, is of httle importance ecologically, since it 
is very short-lived, never reaches any great size, and produces little 
shade. Its hfe history in most respects is similar to that of the 
birch, and it has the same habit of producing sprouts from the 
stump. Pinus Strobus L. (white pine) is scattered thinly through 
many parts of the forest, generally towering high above the other 
trees. Its ecological status seems to be similar to that of the white 
spruce. There is no indication that in recent times at least it has 
ever been abundant upon Isle Roy ale. Picea mariana (Mill) BSP, 
Larix laricina (DuRoi) Koch, and Populus tremuloides Michx., 
which are found here and there in the chmax forest, will be suffi- 
ciently treated in connection with quadrats 5 and 6. Populus 
halsamifera L. also occurs sparingly. 

Taxus canadensis. — The most important of the lower plants 
of the forest — more important indeed than many of the trees — ^is 
the ground hemlock. Its influence lies in the completeness with 
which it occupies and shades the ground, preventing tree reproduc- 
tion over large areas. This effect will be noted in connection with 
quadrats 5 and 6, and quadrat i includes part of a ground 
hemlock area in which trees are practically absent. Taxus can 
endure considerable shading, but is never found in the dense shade 
cast by the balsam groups. Balsam in its turn is excluded from 
large areas by Taxus, so that the competition between these two 
species is exceedingly keen. Taxus spreads abundantly by under- 
ground stems, and in this way invades new areas of forest when 
conditions are favorable, at the same time dying out in the older 
portions of the growth, thus allowing other plants to start in such 
places. 

III. QUADRAT STUDIES 

The method of investigation whereby a knowledge of the dynam- 
ics of the forest was gained was as follows. A rectangular area was 
laid off, made up of one or more units of 5 m. square, the usual 
size being a quadrat of 10 m. square, or four units. In the diagram 
of this area the position and kind of every tree, down to the smallest 
seedling, was plotted and its diameter noted. Cuts were next 
made with an ax to the centers of the large trees, and the small 



24 BOTANICAL GAZETTE [January 

ones were felled. The age of every tree was then determined by 
counting the annual rings, and note was made in each case of the 
degree of soundness of wood, width of rings, and periods of suppres- 
sion indicated thereby. The cuttings were made at the height of 
about 0.3 m. An element of error is introduced here, making 
the age as determined a few years too low. It seemed inadvisable, 
considering the many quadrats to be studied, to use up much 
valuable time in making the counts absolutely accurate. This 
would have involved the cutting of every tree at the surface of the 
ground, a very difficult and slow process. A saw might have been 
used instead of an ax, but when the rings are at all obscure it is 
impossible to count them from a sawed surface. I beHeve that the 
error introduced does not affect the validity of the results, since it 
is approximately the same in nearly every instance. The method 
on the whole gave excellent results, in the study of the rock shores 
and bogs as well as of the climax forest. Its use was made easy 
by the comparatively small size of the Isle Roy ale trees. Sixteen 
quadrats were studied in all, comprising 74 units of area, and 
involving the determination of the ages of about 900 trees. If 
objection be raised that the method is unduly destructive, it may 
be answered that the cutting over of these small areas produces 
exactly the same effect as does windfall, a process that is continually 
taking place, and thus makes possible a new crop. 

In addition to the statistical study of the trees, careful notes 
were taken of the lower vegetation and the physical factors of the 
habitat. Less detailed studies of many other localities were also 
made for comparison with the quadrats. 

The results of the quadrat studies so far as they concern the 
climax forest will now be given in detail. The first four described 
were located on Smithwick Island; quadrats 5 and 6 were upon the 
main Isle Royale. 

Quadrats on Smithwick Island 

Quadrat i (fig. 6). — This quadrat exhibits most clearly the 
relations which the different tree species hold to each other and to 
the physical conditions of the habitat. It includes but one spruce, 
an aged giant 250 years old, long past maturity, with sparse fohage, 



I9I3] 



COOPER— ISLE ROY ALE 



25 



giving practically no shade. Two healthy birches (105 and 107 
years old), close together, produce considerable shade in their 
vicinity. There are several rather old balsams (64-90 years) well 




k^ ^/::^:^^^^®^>^>^ 






Abies balsamea ^ 

Picea canadensis , x 

Betula alba papyrif-era O 

Purus americana U 



5 7 Dead stumps 

3 
2 



Taxus canadensis 

Fig. 6.— Quadrat i, Smithwick Island: the symbols indicate the species; the 
numbers within them the ages of the trees by tens; for example, a tree marked 6 is 
between 61 and 70 years of age. 



scattered over the quadrat, and usually more or less isolated from 
the smaller growth. The young trees are practically all balsams, 
the only representatives of other species being two mountain ash 



26 BOTANICAL GAZETTE [january 

(one a clump of three stump sprouts) and a 20-year old birch in the 
lower right-hand corner. The young growth is not evenly dis- 
tributed, but shows a tendency toward grouping, which tendency 
will be seen in each of the succeeding quadrats. The larger number 
of trees of each group are approximately even-aged. For example, 
the rather scattered group a that surrounds the 5 balsam stumps 
contains 13 balsams, 10 of which are 23-28 years old. Of the 18 
trees in group b, 14 are between the ages of 30 and 50, not so uniform 
as the last, but decidedly of a single generation. Of the 13 trees 
in group c, all but the mountain ash and the large spruce are under 
30 years. 

Group a illustrates in a striking manner the way in which these 
even-aged clusters come into existence. Within its Hmits were 5 
large rotten balsam stumps from which the trees had been broken a 
meter or more above the ground. The group evidently constitutes 
a windfall, probably caused by a single storm, one tree in its fall 
carrying others with it. Such windfalls of various ages are exceed- 
ingly numerous throughout the forest, the balsams, on account 
of their brittleness and susceptibility to fungus attack, being the 
ones most frequently destroyed. This particular windfall is of 
special interest because it was possible to determine the time at 
which it occurred. One of the large balsams in falling pinned to 
the ground a young tree of the same species, which, in spite of 
unnatural position and dense shade caused by the branches of 
the fallen one, has continued to live up to the present. The younger 
tree was 49 years old, and the first 12 rings were exceptionally wide, 
showing that up to the age of 1 2 years it was an unusually vigorous 
sapKng. At this point a sudden change becomes evident, for the 
remaining rings are so close that in counting them a magnifying 
glass was an absolute necessity. This change could have been 
brought about only by some sudden and violent cause, and this 
cause is evidently to be found in the fall of the older tree. The 
windfall is therefore to be dated about 37 years ago. Returning 
to the trees composing group a, we find that they are all balsams; 
one is 85 years old, another 38, a third 14, and 10 range from 23 
to 28; II then are subsequent to the windfall, and 10 began life 



1913] COOPER— ISLE ROYALE 27 

within a period of six years, 9-14 years after the windfall occurred. 
There is only one that clearly antedates it. 

Upon inquiring as to the cause of these facts, the factor of radiant 
energy immediately suggests itself (see p. 20). The older balsams, 
now fallen, when living were close enough to cast a dense shade over 
the area which they controlled, and there can have been no young 
trees beneath them, since if there had been, the present generation 
would antedate the windfall. It was not until 10 years after that 
event that young balsams began to appear in the area. Radiant 
energy being the principal factor involved, this interval of a number 
of years is entirely to be expected, since some time would elapse 
before the disintegration of the tangle of branches with their 
persistent needles would allow a large amount of the energy to 
reach the ground. Evidence in support of this hypothesis was 
found in every quadrat and in every considerable part of the forest. 
In no other case was it possible to determine the exact age of the 
windfall, but the general relation between the older and younger 
generation was usually plainly to be seen. Frequently the only 
sign of windfall is in the rotting moss-covered logs, but the close 
group of even-aged trees, sometimes 50 years old or more, tells the 
story plainly. 

In quadrat i two other, windfall areas are shown, one (c) quite 
recent, the other (b) older. The greater range of age among the 
trees in these areas suggests that those of the former generation 
did not all fall together. This type of windfall is commoner than 
that represented by group a. The fall of the first trees gives the 
wind a better chance to reach others. This slow process may be 
extended over a long period, even until the new generation has 
begun to fill in the gaps first made. In contrast to the dense group- 
ing just described is the remaining area of the quadrat, where the 
individuals are less closely placed and are on the average much 
older. The part not included in the three groups comprises two- 
thirds of the area of the quadrat, yet it contains only 18 trees, 7 of 
which are over 60 years; while the other third of the quadrat con- 
tains 45, only 5 of them being over 60 years. The fewer trees in the 
larger part nevertheless produce a dense shade, and there is very 
little young growth beneath them. 



28 BOTANICAL GAZETTE [january 

There is a difference that should be noted between the shade- 
producing capacity of the balsam and that of the birch. The 
former, with its many whorls of short branches close together and 
its opaque leaves, casts an exceedingly dense shadow which does 
not influence a large area. A moderately close stand of large 
balsams allows extremely little light to reach the ground. The 
birch (in its primeval forest form) influences a large area, but its 
shade is not dense, because of its comparatively thin crown and 
translucent leaves. Under the shade of large birches there is 
frequently a scattering of young growth, while under thrifty 
balsams there is rarely to be found any at all. Both conditions are 
well shown in the diagram of quadrat i. 

The effect of shading is seen also in the undergrowth. In the 
dense shadow of the balsams there is a mere sprinkling of herbs, 
and mosses are usually absent entirely, the ground being covered 
with a layer of tree waste. It is in shade of moderate density and 
in openings that the greatest luxuriance of mosses and herbs is 
found. The ground hemlock is excluded from most of the quadrat 
for the same reason, but in the lower right-hand corner there is an 
area completely occupied by a dense growth of it, which effectually 
prevents the establishment of any other species. 

Quadrat 2 (fig. 7). — This quadrat shows the same features as 
the last. Group a contains a great number of young trees of similar 
age, mostly balsams, which have started as a consequence of one or 
more windfalls. Of the 40 balsams in the area, 33 are between the 
ages of 20 and 35 years, and within these limits, as shown by the 
diagram, there is a tendency for those of similar age to be neighbors. 
Numerous fallen trunks represent the former generation. The 
large balsam marked "12," which was 121 years old — an unusual 
age for this species — was past maturity, and like the big spruce in 
the preceding quadrat was ineffective in producing shade. At 
b there is a part of an older group, 5 of the 7 balsams being between 
44 and 53 years old. The upper left-hand corner is dominated by 
a few old and large trees, balsams and birches, with practically no 
young ones — only a few beneath the birches. The shade in this 
area was dense and the undergrowth sparse, even the ground 



1913] COOPER— ISLE ROYALE 29 

hemlock being nearly absent. No spruce grew in this quadrat, 
but there were occasional large ones near. 

Quadrat 3 (fig. 8). — This small quadrat of one unit area shows 
a group of even-aged trees, among which are several young birch 



^ /^ ^ A ^ 

....---" ^^ AAA 

A A ^ ^ 
A 



A 



A 



A O)^ ^ A / A 





A A 


§ 


57 " 



Abies balsjamea 
Betula alba papyrirei-a 
Pyrus americana 

Fig. 7. — Quadrat 2, Smithwick Island; for explanation of symbols see fig. 6 

and mountain ash as well as balsam. No living trees of a former 
generation are present within the hmits of the quadrat, but several 
were seen near. Decayed trunks were frequent and were mostly 
birch. Of the 37 trees, 26 were within the ages of 26 and 35 years, 
and the others were very close to these limits. Evidently then all 



30 



BOTANICAL GAZETTE 



[JANUAE.Y 



three species started growth at practically the same time, and the 
immediate cause was a windfall. The birches are tall and spindling, 
but now slightly overtop the balsams in spite of a somewhat later 
start. Having gained the advantage as to light supply, their 
tops will spread fast, and these trees, or more Hkely one or two of 
them, will doubtless finally develop the thick-stemmed, spreading, 
round-topped form characteristic of mature specimens in the virgin 
forest. The balsams which are within the sphere of influence of 
the birches will be suppressed. This process in fact has already 

begun. The last 5-10 rings of 
those balsams which were close 
to the birches were found to 
be noticeably narrower than 
the earlier ones, while the 
rings of those growing isolated 
from other trees were uni- 
formly spaced. Undergrowth 
was practically lacking, the 
shade being ever3rwhere very 
dense. Even the ground hem- 
lock was entirely absent. 

In this quadrat the moun- 
tain ash showed an interesting 
habit of growth. Several sap- 
lings were seen among the clos- 
est groups of balsams which were so slender and weak as to closely 
resemble lianes. One specimen was 4.3 m. high and 2.25 cm. 
thick at the base, unbranched, with a single tuft of leaves at the 
top. It was supported entirely by the balsams against which it 
leaned, and its upper portion had penetrated among the interlacing 
balsam branches close to the trunk of a near-by tree. It was 16 
years old, and had evidently started before the balsams had begun 
to shade the ground thoroughly, but was left behind in the severe 
competition for the available Hght supply. 

Quadrat 4 (figs. 9, 10). — This quadrat, also of one unit area, 
includes two generations of balsam and no other species. Several 
large birches were near by and a large spruce. The older generation 




/\bie5 balsamea 
fletula alba papijrifera 
Pijrus americana 



O 



Fig. 8. — Quadrat 3, Smith wick Island; 
for explanation of symbols see fig. 6. 



I9I3] 



COOPER— ISLE ROY ALE 



31 



is represented by one individual in the lower right-hand corner, well 
isolated from other trees, 115 years old. The younger generation 
illustrates competition between individuals of a single species which 
began life at about the same time. Fig. 9 shows the age of each 
tree, and fig. 10 its diameter, which, it may be noted, maintains a 
pretty constant proportion to height. It will be seen from the 
latter that 4 trees (marked by double symbols) have attained a 
much greater size than the others; and if comparison be made with 
fig. 9 it will be evident that these 4 are not noticeably older than 




Abies balsamea 
9 



A 28 



10 



Figs. 9, 10.— Fig. 9, quadrat 4, Smithwick Island: age of trees; for explanation 
of symbols see fig. 6; fig. 10, quadrat 4, size of trees; the numbers indicate the 
diameters of the trees in decimeters. 

their neighbors. The annual rings of these 4 trees were found in 
every case to be wide and evenly spaced, while those of their less 
favored companions were either very narrow from the beginning or 
plainly showed recent suppression. This illustrates an important 
principle in forest study, namely, that no reliance can be placed 
upon the size of a tree in fixing its age or in determining its place 
where two or more generations are concerned. These 4 individuals 
in some way gained the advantage early in life and caused the 
suppression of their neighbors. Evidence of the severity of the 
shading was shown by the presence among the living balsams of 
20 dead specimens, averaging a meter in height, and in length of 



32 BOTANICAL GAZETTE [january 

life from 14 to 38 years. Some had.been dead for a long time, while 
others showed evidence, in needles still clinging, of having been 
alive until very recently. All had undergone severe suppression. 
In consequence of the deep shade the undergrowth was extremely 
sparse, except in the lower left-hand corner where there was a 
partial opening. Here was a luxuriant growth of mosses, including 
Hylocomium proliferum (dominant), Hypnum crista-castrensis, 
Calliergon Schreberi, and Dicranum undulatum. The close group 
of young balsams occupying this locality was largely due to layering. 
Quadrat 4 then includes in the main a group of balsams of very 
different sizes, giving an appearance of gradual reproduction, but 
in reality essentially even-aged, and belonging distinctly to a single 
generation. 

Quadrats on the main Isle Royale 

It has been said that the conditions on Smithwick Island include 
one that is somewhat abnormal for the region as a whole, namely, 
that the exposure to wind is greater. Two quadrats in sheltered 
localities on the mainland of Isle Royale were studied for the 
purpose of comparison. They probably represent the opposite 
extreme so far as exposure is concerned. 

Quadrat 5 (figs. 11, 12) was located a few hundred meters 
back from the southeast shore of the Blake Point peninsula in sec. 
23, T. 67 N., R. 33 W. The locality is thoroughly sheltered from 
northwest winds by the main ridge, and from the lake winds by 
the islands to the southeast. On the diagram several points of 
difference from the preceding quadrats are readily seen. Most 
noticeable are the greater average age of all species and the absence 
of very young growth. Two new trees appear: Picea mariana 
(Mill) BSP (black spruce) and Larix laricina (DuRoi) Koch 
(tamarack), each species being represented by one individual. The 
whole stand is remarkably even-aged, 22 of the 38 trees being 
between the ages of 82 and 98 years. There is some tendency 
toward grouping of trees of similar age, though not so noticeably 
as on Smithwick Island. The group a is a very marked one, 
however. Of the 9 individuals of 4 species composing it, 7 are 
between 83 and 92 years, and 5 between 89 and 92. The effect of 



1913] COOPER— ISLE ROYALE 33 

shelter from wind is very evident in the greater height of the trees. 
The protection which this area enjoys does not by any means 
prevent windfall, but merely lessens its intensity, and allows the 
trees to reach a greater height before they are overthrown. Several 



® 




A 




A ^/ ii^ 


A 


A 


A® ® 
® ® 




A 




A A 




jV ; 




'A 


\ "^ ® U 
, A i 


^ 




\^ ^ ^ \ 


A 







Abies balsamea A 2\ 

Plcea. canadensis v 5 

Belula alba papyrifera Q 13 

Picea manana. D I 

Lanx lancina O f 

Fig. II. — Quadrat 5, Blake Point Peninsula: age of trees; for explanation of 
sjonbols see fig. 6. 

standing dead balsams were seen on this quadrat and they were 
frequent through the neighboring forest. On the outer islands 
the balsams almost never die a standing death. Quadrat 5 (fig. 12) 
also shows suppression of part of the stand, the larger trees being 



34 BOTANICAL GAZETTE [january 

indicated by double symbols. The absence of very young growth 
is explained by the fact that the forest floor was covered by a thick 
mass of ground hemlock. 

Black spruce and tamarack are found occasionally throughout 
the forest. Since they are both very intolerant of shade, they 
probably make a successful start only where an extensive windfall 




Fig. 12. — Quadrat 5 : size of trees; for explanation of symbols see fig. 10 

has let in abundant light. The black spruce as a member of the 
upland forest is abundant in one locahty on the southeast shore 
of Isle Royale and in certain other places. The evidence seems to 
show that this species in abundance in the upland forest indicates 
a transitional stage approaching the climax. The subject will be 
treated again in the consideration of the rock shore succession. 



^913] COOPER— ISLE ROY ALE 35 

Quadrat 6 (fig. 13).— The conditions here as to shelter were 
similar to those of the last. The quadrat was located near the 
shore of Tobin's Harbor in sec. ^^, T. 67 N., R. 2>Z W. The small 
number and large size of the trees are noticeable, and also the entire 




Abies balsamea A 10 

Picea canadensis V I 

Betula alba papurif-era O I 

Hgrus arnericana O 1 

Populus tremuloides Q 6 

Fig. 13.— Quadrat 6, near Tobin's Harbor; for explanation of symbols see fig. 6 

absence of young growth, in spite of the comparative hghtness of 
the shade. These conditions were plainly due to the mat of ground 
hemlock which practically covered the quadrat. The ground 
hemlock therefore, rather than any tree species, dominates and 



36 BOTANICAL GAZETTE [january 

controls this area. The surrounding forest was found to be 
essentially similar to the sample, except that the cover of ground 
hemlock was not continuous, and where it was absent the usual 
conditions of windfall reproduction, especially of balsam, prevailed. 
The large number of good-sized trees, balsams being specially 
noticeable, owe their continued existence to the protected position 
of the area. The presence of Populus tremuloides Michx. (aspen) 
in considerable abundance is noteworthy. This species seems to 
be ecologically equivalent to the birch, except that it does not to 
any great extent possess the power of sprouting from the stump, 
at least in this region. 

A similar situation was noted in a narrow valley near Duncan 
Bay (sec. 28, T. 67 N., R. 33 W.), which was protected by abrupt 
ridges on both sides. Here the trees of all species are very large, 
the shade is not dense, and windfalls are relatively scarce. Ground 
hemlock is exceedingly abundant and large, and is plainly respon- 
sible for the lack of young tree growth and the resulting openness 
of the forest. Some scattered groups of small balsams were plainly 
related to windfalls. 

The foregoing studies show that the climax forest is a complex 
of windfall areas of differing ages, the youngest made up of dense 
clumps of small trees, and the oldest containing a few mature 
trees with little or no young growth beneath, those of a single 
group being approximately even-aged. This mosaic or patchwork 
changes continually in a manner that may almost be called kaleido- 
scopic when long periods of time are considered. The forest as a 
whole, however, remains the same, the changes in various parts 
balancing each other. 

EXTENT OE THIS TYPE OF FOREST AS THE CLIMAX OUTSIDE OF ISLE 

- ROYALE 

• Attempts to obtain information relating to the nature of the 
cHmax forest of other portions of the northeastern conifer region 
have not been attended with much success. The distributions of 
the various trees have been determined by Bell (8) and others 
with considerable accuracy, but practically nothing of an ecological 
nature has been pubhshed. From the data I have been able to 



1 91 3] COOPER— ISLE ROY ALE 37 

discover, the impression has been gained that the same association of 
balsam, paper birch, and white spruce, which is the dominant 
forest type of Isle Royale, is found in the most mesophytic habitats 
throughout northeastern Canada. The probability is that it is 
the climax type over much of the region, though there is not suffi- 
cient evidence to justify a confident statement to that effect. It 
is not necessary that the component species bear the same relations 
to each other in all parts of the region, or even that the species 
themselves be everywhere the same. One or even two of the climax 
trees may be lacking in certain places, species that are ecologically 
equivalent may be substituted, or others added. Analogous differ- 
ences occur in the deciduous forest. The two climax trees that are 
almost omnipresent are the maple and the beech, and yet there is a 
belt along the northern edge of the region where the maple alone 
forms the climax forest, unless the yellow birch (Betula luted) may 
possibly take the place of the beech. Again, in the Great Lakes 
region a third climax tree, the hemlock {Thuja canadensis) is 
present; and in the southern Appalachians the number of species 
composing the climax forest reaches a dozen or more. Similarly, 
in northeastern Canada the climax forest may vary from place to 
place. 

Of northern Quebec Macoun (39) says: "In the country 
around Lake Mistassini it [balsam] grows mixed with aspen, birch, 
and white spruce, and on the lower part of the Rupert River it is 
found growing with the same trees all the way to James Bay." 
The correspondence of this to the Isle Royale forest is striking. In 
reports of the Department of Lands and Forests of Quebec (45, 46) 
expressions such as the following are frequent, the region described 
being the country north of Lake St. John, west to Lake Abitibi: 
"well timbered, mostly with spruce, fir, and white birch, with some 
scattered white and Banksian pine on the high ridges." In the 
"Report of the survey and exploration of northern Ontario" (43) 
there is much detailed information concerning the distribution of 
the trees in that region, though the data presented have Httle 
ecological value. However, in reading the reports of the various 
parties one frequently comes upon such statements as the following: 
"chiefly small poplar [Populus tremuloides], spruce, white birch, 



38 



BOTANICAL GAZETTE 



[JANUARY 



and balsam, and a few balm of Gilead [Populus balsamifera]''; 
"spruce, balm of Gilead, poplar, balsam, and white birch"; "white 
birch, balsam, and a few large spruce"; "the white variety of 
spruce of good size was seen continually along the rivers and on 




, Overlap ot 

Acer saccharym and 
Fagus grandirolla. 



Fig. 14. — Ranges of the climax trees of the northeastern conifer forest and the 
eastern deciduous forest. 



ridges back from them. Black spruce, generally scrubby, clothes 

the muskegs Birch and balsam are also common on high 

lands." The black and white spruces are not usually distinguished 
in these reports, but it is clear that the former is the principal tree 
upon the extensive muskeg lands, while the latter is confined to the 



1913] COOPER— ISLE ROYALE 39 

higher grounds, where it usually has the balsam and birch as its 
companions. 

The data here presented, though very unsatisfactory, are 
sufficient in my opinion to estabhsh the probability of the generah- 
zation that the climax type of the whole of the northeastern conifer 
region is of the general character described for Isle Roy ale, with 
local variations due to the elimination or addition of species, or to 
the substitution of others that are ecological equivalents. 

Important confirmation has recently been received from Dr. 
Robert Bell of Ottawa, the best authority upon the distribution 
of Canadian trees, who writes: ''The same type of upland forest 
which you describe on Isle Royale extends from the Great Lakes 
to James Bay and east and west of it, with modifications in parts." 

On the map (fig. 14) the area shaded with oblique lines repre- 
sents the region over which the ranges of the balsam, paper birch* 
and white spruce overlap, north of the range of the sugar maple 
(data largely from Transeau 55). It is in this region that the 
type of forest described is thought to be the climax. Beyond the 
limits of the balsam, which has the narrowest range of the three, 
some other species must be substituted for it, or else the climax 
forest is composed of the remaining two species alone. 

COMPARISON WITH THE CONIFER FOREST OF THE SOUTHERN APPA- 
LACHIAN SUMMITS 

In connection with the study of the Isle Royale forest it will 
be worth while to make comparison with another region that has 
come under my observation, where the forest is extremely similar 
to that described in the present paper. On the highest summits 
of the mountains of North CaroHna, eastern Tennessee, and south- 
western Virginia, there are isolated areas of dominantly coniferous 
forest, which seem Hke detached portions of the great northeastern 
forest (see detailed description by Harshberger 32). The 
species are different, the balsam being Abies Fraseri (Pursh) Poir., 
the spruce Picea rubra (DuRoi) Dietr., and the birch Betula lutea 
Michx. f. In general aspect this forest is surprisingly hke that of 
Isle Royale. Because of the predominance of the first tree men- 
tioned many of the mountains themselves are locally called "Bal- 



40 BOTANICAL GAZETTE [january 

sams." The results of studies upon three of these summits in 
western North Carolina may be briefly summarized as follows, 
the localities being Richland Balsam, Plott Balsam, and the Black 
Mountains (Mount Mitchell). 

The coniferous forest covers the mountain slopes from about 
1600 m. to the summits, the highest of which is about 2010 m. 
Abies Fraseri is on the whole the most abundant species except 
along the lower edge of the coniferous region, where Picea rubra 
is of somewhat greater importance. Betula lutea is scattered more 
or less thickly throughout and grows to a great size, specimens 
having been noted that were 1.3 m. in diameter. As on Isle 
Royale, birch and spruce are sparsely represented in the young 
growth, which is predominantly balsam. The shrubby vegetation 
consists almost entirely of Rhododendron catawbiense Michx., which 
is very abundant. It is interesting to note that the balsam seed- 
lings are practically absent under the shade of the rhododendrons, 
and scarce in shade in general, but are exceedingly abundant in 
partial openings. The ground is covered by a luxuriant moss 
carpet, almost identical in composition with that of the Isle Royale 
forest, and the herbaceous growth includes most of the character- 
istic group of northern forest plants which has been hsted (p. 16). 

The similarity between Isle Royale and the North Carohna 
"Balsams" is thus a striking one. In the latter region there is 
even an ecological equivalent to the ground hemlock. Rhododen- 
dron catawbiense, in spite of its very different habit, is equally 
effective in densely occupying and shading the ground and thus 
in temporarily preventing reproduction of the forest trees over wide 
areas. I believe that the conclusions which have been reached 
concerning the Isle Royale forest will also hold, with minor modi- 
fications, for the forests of the North Carohna summits. The 
conifer-birch forest of the mountains is to be regarded as the cHmax 
type of its own limited area (not including, of course, the lower 
slopes dominated by deciduous trees), and at the same time as an 
extension or outlier of the northeastern climax forest. 



1913] COOPER— ISLE ROYALE 41 

THE MAPLE FOREST OF ISLE ROYALE AND ITS RELATIONS 

Wherever the sugar maple occurs it forms a part of the climax 
forest, and is usually the dominant species therein. Between the 
two great eastern forest regions there is a transitional belt several 
hundred kilometers wide where the three climax trees of the conifer, 
and the two of the deciduous forest all occur (see map, fig. 14)- 
This belt extends from northern Wisconsin through the upper 
peninsula and the northern part of the southern peninsula of Michi- 
gan and eastward to New Brunswick. Whitford (59) studied the 
successions in a portion of this belt, and found that the climax 
forest in northern Michigan (both peninsulas) is the beech-maple 
type. The balsam, birch, and spruce are very abundant, but here 
they belong to preliminary stages in the successions. Ganong 
(26, 27) gives an excellent summary of the plant formations of 
New Brunswick. He states that the climatic forest type is the 
"mixed maple-birch-spruce-fir association." There is no indica- 
tion in his paper that the maple ever supersedes the other trees, 
but the presence of such a possibiKty must be admitted. The con- 
clusion from the studies of Whitford and others seems to be that 
the maple and beech, where not climatically excluded, are able to 
supersede the climax trees of the northeastern forest. 

Coming now to Isle Royale, we find upon the southwestern end, 
occupying the summit of the highest ridge, a mixed growth of 
Acer saccharum Marsh (sugar maple), Betula lutea Michx. f. (yellow 
birch), and B. lenta L. (sweet birch); with the characteristically 
northern trees as a minor element (see Adams 4, PP- 30-31 > and 
Holt 33, p. 224). The maple is decidedly the dominant species 
and reaches a large size.^ At the northern edge of this northernmost 
outpost of the maples we may draw the line that separates the true 
northern forest from the transitional belt (fig. 14)- South of this 
fine the representatives of the southeastern deciduous forest, 
though not necessarily forming the bulk of the stand, yet have the 
upper hand; north of it the supremacy of the conifers and the paper 
birch is undisputed. 

I It is of interest to note that the maple is common on the southern side of Michipi- 
coten Island, near the eastern shore of Lake Superior. 



4"2 BOTANICAL GAZETTE [January 

Since glacial times there has been a continual northward advance 
of the forest, with the conifers as the pioneers, closely followed by 
the hardwoods. The problem as to whether the extension of the 
latter is still going on might be studied to good advantage in such 
locaHties as the southwestern end of Isle Royale, and Michipicoten 
Island. The large size and thriftiness of the maple at its northern- 
most hmit would seem to indicate that it has not reached its climatic 
limit (Bell 8). The manner and causes of "climatic successions," 
or the invasion of one climax forest by another, are still to be worked 
out. 

SUMMARY. — THE CLIMAX FOREST 

I. The dominant forest of Isle Royale is composed of Ahies 
halsamea, Betula alba var. papyrifera, and Picea canadensis, with a 
few other species occasionally present. Abies, all sizes and ages 
considered, is by far the most abundant, but the greater number of 
individuals are small. Betula, although conspicuous, is not abun- 
dant, and young trees are scarce. Picea is rare, though occasional 
specimens tower high above the other trees. Shrubs and herba- 
ceous growth are sparse except in partial openings. The most 
important element in the latter is the moss contingent, which is 
responsible for the formation of great amounts of humus. 

II. Studies of individual species gave the following results. 
Abies is preponderant in the young growth because (i) the seedhngs 
make a successful start in almost any situation provided sufi&cient 
light be available; (2) the species reproduces abundantly by layer- 
ing. Its rapid decrease when greater size and age are considered 
is due to (i) competition because of abundant germination; (2) 
fungus attacks, and (3) brittleness of wood, both resulting in 
extreme liability to windfall. Its high birth-rate is balanced by a 
high rate of mortality. Betula does not germinate abundantly in 
the forest, but, because it is not hable to disease and windfall, 
holds its own with Abies. Even when broken off by severe winds 
it has a means of recovery in its ability to produce stump sprouts. 
Its more rapid growth gives it an advantage in competition with 
Abies. Abundant light is necessary for successful reproduction. 
Low birth-rate is compensated by a very low mortality. Picea is 



1 913] COOPER— ISLE ROY ALE 43 

ecologically unimportant on account of its scarcity. Germination 
in the forest is less abundant than in the case of Betula. It is not 
liable to fungus attacks and withstands severe winds. Birth-rate 
and mortality are both low. Taxus canadensis is the most impor- 
tant species of the undergrowth, its influence lying in the complete- 
ness with which it occupies and shades the ground, preventing tree 
reproduction over large areas. 

III. Intensive study of selected areas (quadrats) yielded the 
following facts concerning the dynamics of the forest. 

The forest is a complex of windfall areas of differing ages, the 
youngest made up of dense clumps of small trees, and the oldest 
containing a few mature trees with little young growth beneath. 
The history of a windfall area is as follows. After the debris 
has disintegrated sufficiently to allow abundant light to reach the 
ground, a new generation of trees springs up, approximately even- 
aged, composed of the three dominant species, Ahies always greatly 
preponderant. During the continued development of this group 
most of the individuals are at various times ehminated, Ahies suffer- 
ing most for the causes enumerated in section II. Because of the 
dense shade no new individuals can start beneath them, and the 
final outcome is a group composed of a few large trees, approxi- 
mately even-aged, in which Ahies has nearly or quite lost its posi- 
tion of dominance to Betula. In situations sheltered from wind 
all species live to a greater age and windfalls are less frequent. The 
processes though less rapid are nevertheless the same as in more 
exposed situations. The result in the forest in general is a mosaic 
or patchwork which is in a state of continual change. The forest as 
a whole remains the same, the changes in various parts balancing 
each other, 

IV. The following evidences that the dominant forest of Isle 
Royale is also the climax have been derived from the studies sum- 
marized above and from those dealing with the successions. 

1. The dominant forest is the most mesophytic of the plant 
societies. 

2. It is uniform upon all soils and upon areas that have passed 
through very different lengths of subaerial history. 

3. All the successions culminate in the estabhshment of this as 
the final stage. 



44 BOTANICAL GAZETTE [january 

4. The character of the forest as a whole is stable, though an}^ 
given area is continually changing in composition and relative 
proportions of the various species. 

V. The same type of forest, with local differences in some 
places, is probably the climax throughout the northeastern conifer 
region. 

VI. Comparison with the conifer-birch forest of the southern 
Appalachian summits shows a striking equivalence of species and 
marked correspondence between the two in ecological character- 
istics, indicating that the forest dynamics are essentially the same. 
The mountain forest may logically be considered as a southward 
extension or outlier of the northeastern climax forest. 

VII. Acer saccharum is dominant upon the main ridge at the 
southwestern end of Isle Royale, reaching here its extreme northern 
limit in this region. Southward it is probably able to supersede 
the conifers and birch, while north of its northern limit the suprem- 
acy of the latter trees is undisputed. 

Palo Alto, California 



=M 



ISLE R OVALE 



Lake Superior 

michigan 
Adapted from Lane 



h 



LIBRARY OF CONGRESS 



000 880 109 5 # 




